Joe Curry wrote:
>I don't think so. These are scientific principles at work here.
>
>Compression causes heat (That is why intercoolers are so popular to
>cool the air that is compressed by superchargers and turbochargers)
>
>Heat typically causes expansion.
>
>The two principles are irrefutable. Now, I won't say that in
>hydraulic systems the principles apply to any great extent because
>as I said, the fluids
>are selected due to their properties that resist compression. And
>since the action at the input is reciprocated by an action at the
>other end of the
>system, not much compression takes place anyway. It is more an
>action of displacement rather than compression.
I'm totally with Joe on this one
however, I can understand why
people on a purely knee-jerk level perceive this as a contradiction.
The expansion in the fluid that takes place as a result of heat
(which results from pressure) is on a molecular level. There really
is no 'real-world' effect of that expansion
it is so small that it
can basically be written off as purely theoretical. Therefore, the
expansion does not negate or even diminish (significantly) the net
effect of the action of the hydraulic system.
Joe also correctly points out that hydraulic fluids are not supposed
to be compressible (relatively speaking, that is). If hydraulic
fluids were compressible, hydraulic systems wouldn't function.
There is really very little compression going on in a clutch
hydraulic system (for instance) because there is something on the
other end allowing fluid to move. This is what Joe refers to when he
says 'displacement.' Braking systems, however, are different as the
pistons in the caliper stop allowing fluid to 'move' as soon as the
pads contact the rotor, and yet still we push harder and harder on
the pedal. Since the piston on the other end has met the rotor, and
since the fluid is NOT compressible, all we accomplish by continuing
to push is to increase pressure. This creates heat in addition to
the heat that is created at the pads, and although the heat from the
pads is far greater in proportion, all of the heat involved will
contribute to fluid fade. Pad fade is only the result of heat at the
pads, but fluid fade is the result of all the heat created.
Joe's propensity to type 'fluif' instead of 'fluid' has not hurt the
man's credibility on hydraulics in my book!
Give 'em Hell, Joe!
--
Pete Chadwell
1973 TR6
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